This collaborative research program represents efforts to gain new insight into critical events which determine the outcome of an encounter between antigen and CD4 cells. Dr. Ploegh's group will further define the intracellular pathways travelled by class II molecules leading to peptide- loading and delineate the interaction of peptide-loaded class II molecules with the TCR (Project 1). Ligation of the TCR by class II/peptide complexes on APC can result in clonal expansion and T-cell memory or in transient activation followed by T-cell elimination. Dr. Cantor's group has recently delineated a TCR-coupled signalling pathway associated with transient activation and apoptosis. The proposed studies will more fully characterize the intracellular biochemical events which comprise this TCR- coupled pathway and delineate their relationship to programmed cell death (Project 2). Although there is good evidence that efficient clonal expansion of CD4 cells may require signalling from the CD28 coreceptor, the biochemical basis of this phenomenon is not well understood. Dr. Rudd's group has obtained evidence that PI-3 kinase and the Grb2/mSOS signalling complex interact within the same site on the cytoplasmic tail of CD28. He will define the stoichiometry of this interaction as well as its implication for CD28-dependent activation of T-cells (Project 3). After a successful encounter of CD4 cells with antigen, commitment to the TH1 or TH2 lineage is regulated in part by IL-4. Dr. Glimcher's group proposes to define the molecular basis of IL-4 gene expression based on their recent definition of an important regulatory segment within the IL-4 promoter region (Project 4). The biochemical, genetic and cellular expertise brought to bear on these issues should yield new information on the molecular basis of peptide presentation to the TCR, signalling pathways leading to clonal expansion or apoptosis, and the genetic mechanisms responsible for the development of functionally distinct Th cells. This basic information is essential to design approaches leading to long-term protective immunity following vaccination.